The present invention relates to an electrochemical device and a method for operation thereof.
There are fuel cells that depend on methanol for fuel. Such fuel cells are divided into two categories according to the mode of decomposition of methanol. (See FIG. 2). Those in the first category work in such a way that methanol reacts with water for decomposition into hydrogen gas, and the hydrogen gas is supplied to the negative electrode of the fuel cell. These are referred to as fuel cells of the methanol reforming type. Fuel cells in the second category work in such a way that methanol reacts directly with water on the negative electrode without conversion into hydrogen gas. These are referred to as fuel cells of the direct methanol type.
Fuel cells of the direct methanol type offer the advantage of storing methanol in liquid form. Therefore, it is superior in efficiency and safety to the fuel cell of hydrogen type which stores hydrogen gas. In addition, fuel cells of the direct methanol type can be readily made portable owing to their simpler structure relative to those of the methanol reforming type because they do not need a gas reformer.
Furthermore, in order for fuel cells to be more practicable, it is necessary to reduce their size and simplify their construction. This object is not achieved by those fuel cells which need high temperatures in excess of 200xc2x0 C. for their operation or which employ a liquid electrolyte. By contrast, it is expected that this object can be achieved by those fuel cells which employ a polymeric solid electrolyte and operate on the basis of the direct methanol method. However, such fuel cells nonetheless have problems as explained below.
A fuel cell of direct methanol type which employs perfluorosulfonic acid resin (such as Nafion (from DuPont) as a polymeric solid electrolyte in membrane form has already been developed. This polymeric solid electrolyte requires the existence of a large amount of water within the polymeric solid electrolyte for proton conduction when the fuel cell operates. Unfortunately, fuel cells of this type have the disadvantage that a large amount of methanol together with water enters the electrolyte membrane from the negative electrode (fuel electrode). This methanol eventually reaches the positive electrode (bringing about cross-over), resulting in a decrease in voltage or output and fuel efficiency.
Another problem is that fuel cells of direct methanol type need a higher catalytic capacity although there are no catalysts meeting this requirement. One practical way of supplementing the catalytic capacity is to heat the operating temperature up to about 150xc2x0 C. However, heating is not practicable for the polymeric solid electrolyte (such as Nafion) whose operating temperature is limited to 80xc2x0 C. because of its characteristic properties.
It is an object of the present invention to provide an electrochemical device and a method for operation thereof. The electrochemical device is applicable to fuel cells employing a solid electrode which works at temperatures of 200xc2x0 C. or lower in combination with the direct methanol method. The operation of the electrochemical device is intended to increase voltage and output and improve fuel efficiency of the fuel cell without cross-over. Further, the electrochemical device permits selection of an electronic catalyst from a broad range of electrode catalysts.
To achieve the above object, according to an aspect of the present invention, an electrochemical device is provided including: a first electrode, a second electrode, and a proton conducting material held between the first and second electrodes. A major component of the proton conducting material contains a fullerene derivative having proton-dissociating groups introduced into the carbon atoms of fullerene. The term xe2x80x9cproton-dissociating groupxe2x80x9d means functional groups capable of dissociation into proton (H+) by electrolytic dissociation. (The same shall apply hereinafter). The first electrode permits reaction between methanol and water for proton generation.
The present invention further provides a method of operating an electrochemical device having a first electrode, a second electrode, and a proton conducting material held between the first and second electrodes, wherein a major component of the proton conducting material is a fullerene derivative having proton-dissociating groups introduced into the carbon atoms of fullerene. The method includes the step of supplying the first electrode with methanol and water to produce a reaction to generate protons.
The electrochemical device according to the present invention is a novel design in which the direct methanol method is ingeniously combined with the above-mentioned fullerene derivative. The electrolyte containing mainly the fullerene derivative exhibits proton conducting ability without resort to water as the transfer medium. This minimizes the amount of water consumed by the reaction with the methanol and permits the electrolyte to be compactly formed. Thus, only an extremely small amount of aqueous solution of methanol moving toward the positive electrode exists.
A consequence of the foregoing is the substantial absence of cross-over, increased voltage and output, and improved fuel efficiency. In addition, the above-mentioned electrolyte permits the fuel cell to operate at 150xc2x0 C. or above. This permits selection from a broad range of catalysts or the improvement of existing catalysts in their catalytic capacity.